Process for producing metal flakes

ABSTRACT

The present invention provides a jetting process for the production of flakes with uniform size distribution to be used in pigments comprising the steps of ejecting molten metal from a jet head and collecting droplets of metal on a solid collecting substrate or collecting droplets of metal in or on a collecting substrate.

This application is a divisional of U.S. application Ser. No. 11/990,077filed Feb. 13, 2008, which in turn is the U.S. national phase ofInternational Application No. PCT/GB2005/003168, filed 12 Aug. 2005, theentire contents of each of which are hereby incorporated by reference.

The present invention relates to a jetting process. The presentinvention further relates to flattened metal particulates, processes fortheir production and their use, especially as functional fillers andpigments.

The term atomised metal powders is used in the industry to implysomewhat spherical particulates. They are thereby distinguished fromflattened metal particulates, especially those flattened metalparticulates of the metal pigment industry, which are generally referredto as flakes.

Amongst metal flakes, aluminium and gold bronze (an alloy of copper andzinc) are the most widely manufactured, but copper, gold, iron, nickel,silver, stainless steel, tin, and zinc flakes are all commerciallyavailable. Applications of aluminium and gold bronze pigments are mainlyfor silver or gold coloration respectively of paints, inks, powdercoatings and plastics. These and copper, gold, iron, nickel, silver,stainless steel, tin, and zinc metal flakes may also have functionalproperties, such as electrical conductivity, heat and light reflection,moisture barrier or flame retardancy.

The preparation of metal flake pigments from conventionally atomisedmetal powders is well documented in the patent literature. Such pigmentsmay be prepared in the complete absence of solvent by a dry ball millingprocess, but this can be hazardous in the case of reactive metals suchas aluminium, due to the contaminating and/or explosive properties ofthe dry flake products. For such metals, dry milling has been largelysuperseded by wet ball milling processes in which metal powder is milledwith an organic liquid such as mineral spirits and a small amount of alubricant. The cascading action of grinding media within the ball millcauses the substantially spherical metal powder to be flattened out intoflakes having aspect ratios (the ratio of the largest dimension to thesmallest) of from about 15:1 to around 150:1 or even up to 250:1 andbeyond. The function of the lubricant is to prevent the cold welding ofadjacent flakes that can occur under the impact of grinding media.

Older production processes produce flakes with angular edges and unevensurfaces, known in the art as “cornflakes”. A more recent developmentrelating to aluminium is so-called “silver dollar” flakes. These aredistinguished by more rounded edges, smoother, flatter surfaces andoften a narrower particle size distribution. In consequence they havebrighter, whiter and more desirable appearance than cornflakes.

Irrespective of the method of milling, the most common starting materialis atomised metal powder. Atomised metal powders are prepared by meltingthe bulk metal then continuously forcing it through a nozzle by means ofhighly compressed gas. In this way, bulk metal is converted to aplurality of fine powder particles, whose shape, median particlediameter and particle size distribution are determined by such factorsas metal type, atomising temperature and the compressed gas type andpressure. Atomisation processes have been known for many years. Forexample, an early apparatus is described in U.S. Pat. No. 1,306,060. Thenature of the gas used greatly influences the geometry and surfacefinish of the derived powder. Thus air atomisation results in powderhaving both irregular shape and surface, whereas an inert gas such asargon or nitrogen provides substantially spherical powder with asmoother surface.

Such atomising processes share two common features. Firstly, it istechnically difficult to produce high yields of metal powders withmedian particle diameters less than about 10 μm. Such fine powders areparticularly sought after for powder metallurgy, rocket propellants andfor the production of metal flake pigments of high hiding power(opacity). In practice, this fine particle size fraction, which mayaccount for no more than 20% of the atomised material, is separated fromcoarser powder by sieving or centrifugal classification. In the case ofsome reactive metals, such as aluminium, this operation must beconducted in an inert atmosphere, to avoid the possibility of explosion.

The second characteristic exhibited by conventional atomisationprocesses is the generation of powder with a wide particle sizedistribution. A typical distribution is shown in Table 1, whichillustrates a commercially available aluminium powder with a medianparticle diameter (D₅₀) of 39.63 μm, a D₁₀ of 14.41 μm and a D₉₀ of80.78 μm. Thus the span, defined as the D₉₀ less the D₁₀, thereafterdivided by the D₅₀, is 1.67, characterises this typically broad particlesize distribution.

The twin limitations of fine powder production and wide particle sizedistribution have a consequential limiting effect on the characteristicsof metal flakes prepared from atomised powders.

Narrow particle size distribution is a highly desirable characteristicof metal flake pigments. Excessively fine flakes, such as those having adiameter of up to about 2 μm, have increased opacity or hiding power,but are substantially less brilliant than larger flakes. Coarser flakes,for example those with a diameter over 30 μm, may exhibit an undesirabledegree of sparkle in modern coatings, in which a smooth visual effect isdesired. Large flakes have substantially reduced opacity and can alsocause problems in application. For example, when used in inks, they mayblock the cells of gravure printing presses.

To obtain a narrow particle size distribution, it is customary tosubject the products of milling to size segregation procedures, such asdry or wet screening, to separate the desired particle size range fromexcessively large and/or small flakes. In this way a brighter, smootherand more desirable metallic appearance is obtained. Such a sizesegregation process adds to the cost of the desired product, as theremoved material may have either no, or limited commercial use. Inpractice, oversize material is often returned to the milling apparatus,to be further ground down into the desired particle size range.

U.S. Pat. No. 5,266,098 describes a process for making uniformly sizedmetal droplets. The process uses a container capable of holding moltenmetal with at least one orifice where this metal can be ejected, alongwith a vibrating mechanism and pressure to force the molten metalthrough the orifice, and a means of providing an electrical charge onthe droplets so formed. The cooled droplets have a uniform size, varyingby only about ±25% from the average.

Means to prepare fine metal powders of narrow particle size distributionare described in, for example U.S. Pat. No. 5,810,988. This describes ajetting apparatus to produce and collect uniformly-sized, metaldroplets, comprising a droplet generator, disposed to emit molten metaldroplets in a generally upward direction to cause emitted droplets totravel a parabolic path, during which they are cooled withoutdeformation to form substantially spherical solid metal droplets. Thatinvention further provides a jet control system to electromechanicallygenerate a stream of the molten droplets by either continuous ordrop-on-demand (DOD) methods. Envisaged applications for the solid metaldroplets include soldering of printed circuit boards, deposition ofmicrocircuits and patterns, mask making, circuit board repairs andfabrication of microwave circuit components.

In U.S. Patent Application 2003/186485A1 there is described a jetapparatus consisting of a head having a plurality of ejection ports,each of which is associated with a gas ejection conduit connected to achamber in which hydrogen gas may be instantaneously generated by achemical reaction. This gas provides the pressure to eject molten metalfrom the ejection ports. The invention is directed to preparation ofsolder bumps on semiconductor dies.

There is a need for a metal powder that has both a low median particlediameter and a narrow particle size distribution. By using such a powderin the preparation of metal flake pigments, a substantially monodispersemetal flake pigment product may be obtained in virtually 100% yield,i.e. without the necessity of removing over-sized and/or under-sizedflakes for re-milling or recycling. Both the economics of such a processand the colouristic attractiveness of such a product are considerable.

As described above the most common starting material in the preparationof metal flakes is atomised metal powder. This is prepared by meltingthe bulk metal then forcing it through a nozzle by means of compressedgas. Thus bulk metal is converted to powder only to be further workedmechanically in the ball mill to form flakes. There therefore exists theneed for a process that converts bulk metal directly to flakes.

DISCLOSURE OF THE INVENTION

Process

In a broad aspect, the present invention provides a jetting processcomprising the steps of ejecting molten metal from a jet head and either(a) collecting droplets of metal on a collecting substrate, wherein thecollecting substrate comprises a solid release layer; or (b) collectingdroplets of metal in or on a collecting substrate and subsequentlymilling the collected metal droplets.

The term “release layer” means a material to which the droplets of metaldo not become permanently attached. This allows the droplets of metal tobe detached from the release layer for example by mechanical means or bywashing the release layer with a suitable recovery liquid. The term“solid release layer” means a release layer that is sufficiently solidto cause at least some degree of deformation to the metal droplets.

In a further broad aspect, the present invention provides a process ofpreparing flattened metal particulates, the process comprising the stepsof ejecting molten metal from a jet head and either (a) collectingdroplets of metal in the form of metal flakes on a collecting substrate,wherein the collecting substrate comprises a solid release layer; or (b)collecting droplets of metal in the form of metal particulates in or ona collecting substrate; and subsequently treating the metal particulatesto provide flattened metal particulates.

The term “flattened metal particulate” refers to metal particles whichare not spherical, i.e. which have an aspect ratio of at least 1.1:1wherein the aspect ratio is defined as the ratio of the largestdimension to the smallest dimension. In one preferred aspect theflattened metal particulates have aspect ratios of at least 1.5:1. Metalflakes are a particularly preferred type of flattened metal particulate.

The term “metal flake” as used herein refers to a metal particle havingan aspect ratio of at least 5:1. According to a preferred aspect of theinvention the metal flakes are substantially cylindrical and the aspectratio is then the ratio of the diameter to the height.

In process (b) of the invention, the metal particulates may be treatedby one of the processes described in U.S. Pat. No. 5,593,773 to formflattened metal particulates. These processes include ball milling atlow ball collision energies for example by slow speed ball milling witha low ball charge, or by increasing the viscosity of the mill base inthe ball milling regime appropriate to conventional flake formation. Inthis embodiment the flattened metal particulates can be regarded asslightly distorted, or facetted, spheres which have a greater surfacearea and hence reflectivity than the untreated metal spheres from whichthey are produced. U.S. Pat. No. 5,593,773 is herein incorporated byreference.

In a preferred embodiment, in process (b) of the invention, the metalparticulates are milled to provide metal flakes.

Thus in one aspect the present invention also provides a process ofpreparing metal flakes, the process comprising the steps of ejectingmolten metal from a jet head and either: (a) collecting droplets ofmetal in the form of metal flakes on a collecting substrate, wherein thecollecting substrate comprises a solid release layer; or (b) collectingdroplets of metal in the form of metal particulates in or on acollecting substrate; and subsequently milling the metal particulates toprovide metal flakes.

In one aspect the present invention provides a process for preparingmetal flakes directly from the bulk metal, thereby avoiding theexpensive conventional steps of atomised powder preparation followed byball milling.

In another aspect, the present invention provides a process forpreparing metal flakes of narrow particle size distribution fromsubstantially spherical metal particulates of much narrower particlesize distribution than are obtainable by conventional atomisationtechniques.

The metal may be melted in any crucible or reservoir recognised for thepurpose, providing that there is good thermal control.

Preferably the molten metal is supplied to the jet head from areservoir. A short path from the crucible or reservoir to the jet headcontaining the nozzle plate and nozzles is desirable. This is to avoidexcessive cooling of the mass prior to entering the nozzle, which couldlead to blockage thereof.

The mechanism by which the jet head ejects molten metal is not critical,providing sufficiently small droplets can be uniformly generated at highspeed and the materials of construction are capable of withstanding themelt temperature of the metal in use. In a preferred aspect of thepresent process, the molten metal is ejected vertically downwards.

Jet heads of the continuous ink jet (CIJ) and drop-on-demand (DOD) typesare especially amenable to the process of the invention, provided thatthe piezoelectric control of the latter is kept well insulated and sowell below the temperature of the molten metal. Those jet headsoperating via piezoelectrically controlled nozzle elements are generallysuitable for metals and alloys melting below the Curie point. Above theCurie temperature, the piezoelectric mechanism becomes inoperative. TheCurie temperature for one of the commonest piezoelectric materials, leadzirconium titanate (PZT), is around 350° C.

An effective control mechanism is essential for the production ofuniform droplets. Inducing droplets to part reliably and uniformly onegress from the nozzle entrance may be achieved via a number ofmechanisms. These include, but are not limited to, pulsed RF heating,pulsed laser heating, acoustic standing waves and electrically inducedheating of the metal itself, based on its inherent resistance. Not alltechniques will be applicable to all metals, but the optimum mechanismmay readily be determined by experiment.

Design of the nozzle plate and its nozzles is a further importantparameter in controlling the nature and uniformity of the droplets. Inpractice, a nozzle plate will accommodate a plurality of nozzles;perhaps several hundred. In this way, blockage of a few nozzles does notcritically impair overall performance. A plurality of nozzles isdesirable in order to prepare commercial quantities in a reasonabletimescale.

Nozzle orifices may have a circular section, or may be of othergeometries. Diameters may range from 2 μm to 100 μm or more, dependingon the desired metal flake size. It has been found that small diametersilicon wafers are effective materials for nozzle plates, being of goodchemical resistance and durable if well supported. Laser drilled rubycrystals have also been successfully employed. The relationship betweennozzle dimensions and droplet size depends on the many operatingparameters, but a droplet diameter range of up to 3 times the smallestnozzle dimension is generally found.

A filter for the molten metal before it enters the nozzles is alsoadvantageous. Filtration of the molten metal is recommended to preventnozzle blockages which may occur, for example, if there are impuritiessuch as oxide or slag, in the molten metal.

Filtration may be achieved by any filters routinely employed for moltenmetals. The pore size of the filter is however desirably a maximum ofhalf the nominal nozzle diameter. In practice, it has been foundsatisfactory to install a series of filters of successive finenesswhereby the filter that has a maximum of half the nominal nozzlediameter is the last to be encountered before the molten metal reachesthe nozzle. Ceramic filters of pore sizes down to 5 μm have beenadvantageously used.

The metal is preferably selected from aluminium, zinc, copper, tin,nickel, silver, gold, iron and alloys thereof such as gold bronze, analloy of copper and zinc, alloys of tin with aluminium, gallium and/orindium, or stainless steel, an alloy composed mainly of iron. Morepreferably, the metal is selected from aluminium, zinc, copper, tin,nickel, silver and alloys thereof such as gold bronze. In one aspect,aluminium, copper, tin, silver and alloys thereof are particularlypreferred. In another aspect aluminium, tin and gold bronze areparticularly preferred. In a further aspect, aluminium and gold bronzeare particularly preferred.

In order to prevent oxidation of the more reactive metals and thepossible formation of oxide slags and also to reduce explosiveness, theprocess of the invention may advantageously be carried out under aninert gas atmosphere.

According to one aspect of the invention, the jet head and thecollecting substrate are in a chamber under an inert atmosphere, orunder at least a partial vacuum. The inert atmosphere is preferablyprovided by an inert gas such as nitrogen. Use of an inert gas or atleast a partial vacuum is advantageous because it limits or avoidsoxidation of the molten metal, the metal flakes and/or the metalparticulates. In an alternative embodiment the chamber could be filledwith air.

In one preferred aspect the gas in the chamber is chilled. In anotherpreferred aspect the gas is directed vertically upwards. These preferredaspects may advantageously be used to limit the height of the chamberrequired.

An elevated pressure applied to the crucible containing the moltenmetal, and/or a vacuum applied to the nozzle discharge side of theapparatus may also aid reliable ejection from the nozzles.

Process (a)

According to a first embodiment the present invention provides a processof preparing metal flakes, the process comprising the steps of ejectingmolten metal from a jet head and collecting droplets of metal in theform of metal flakes on a collecting substrate, wherein the collectingsubstrate comprises a solid release layer.

This embodiment may advantageously be used to prepare metal flakesdirectly from the bulk metal without the need to convert the bulk metalto a powder which then needs to be further worked mechanically to formflakes.

The metal flakes generally have a substantially circular face but may bedifferent shapes depending on the intended application. For example themetal flakes may have a substantially triangular, square, rectangular orstar-shaped face or may be rod-like or fibre-like.

Circular shapes may be obtained from single droplets whereas othershapes are obtained by directing two or more droplets onto thecollecting substrate such that the second and subsequent dropletsimpinge partially on earlier droplets and fuse therewith. The thicknessand surface characteristics of the metal flakes may be controlled byadjusting the contact angle and surface tension relationship between themolten metal and the collecting substrate material. The optimumoperating conditions for a given combination of metal and collectingsubstrate may be determined by routine experimentation.

Rapid cooling takes place as the droplet of metal leaves the jet headand impinges on the collecting substrate. According to process (a) ofthe present invention, the distance between the jet head and thecollecting substrate is selected such that the droplets of metal remainsufficiently fluid at the point of impact with the substrate to formmetal flakes. Various factors need to be taken into account in order toselect an appropriate distance. These factors include the nature of themetal, the molten metal temperature in relation to its melting point,the droplet size, the collecting substrate temperature and theatmospheric temperature in the region of the jet head.

Preferably the distance between the jet head and the collectingsubstrate in process (a) is less than 0.30 metres, such as less than0.25 metres or less than 0.20 metres. More preferably the distancebetween the jet head and the collecting substrate in process (a) is from0.001 to 0.01 metres.

Thus, preferably this first embodiment provides a process of preparingmetal flakes, the process comprising the steps of ejecting molten metalfrom a jet head and collecting droplets of metal in the form of metalflakes on a collecting substrate, wherein the collecting substratecomprises a solid release layer and wherein the distance between the jethead and the collecting substrate is less than 0.25 metres.

In one aspect the solid release layer has a low friction coefficient.This property is advantageous because it allows the metal flakes to bereadily removed from the solid release layer. Solid release layershaving a low friction coefficient include polytetrafluoroethylene (PTFE)and silicon. Alternative solid release layers include polished glass orpolished ceramic.

The solid release layer should be made of a material that is able towithstand the temperature of the metal droplets at the instant ofcontact. The distance between the jet head nozzle and the solid releaselayer will affect the temperature of the metal droplets at the instantof contact and hence the suitability of a particular solid releaselayer. The thermal limitations of PTFE will typically restrict itssuitability to metals and alloys having low melting points.

One example of a release layer is a layer of organic or inorganic,solvent soluble or solvent dispersible material which is coated onto asubstrate, generally as a solvent solution. The solvent is subsequentlyremoved therefrom, for example by evaporation. Metal flakes which aredeposited onto such a release layer may be subsequently removedtherefrom by redissolution or redispersion of the release layer in thesame or another solvent. The thermal limitations of such release layersmay restrict their suitability to metals and alloys having low meltingpoints.

In one preferred aspect the solid release layer is a continuous belt oris on a continuous belt. For example the solid release layer may beaccommodated on a continuous belt by division into strips. Preferablythe solid release layer is kept in motion to control the spatialdisposition of the metal flakes and in this aspect a continuous belt isparticularly advantageous. A continuous belt also allows the metalflakes to be released from the solid release layer downstream of the jethead nozzles. When the metal droplets are milled prior to release fromthe solid release layer a continuous belt is also advantageous since themetal droplets may be milled downstream of the jet head nozzles whilststill on the belt, solidified and then released from the solid releaselayer further downstream. A continuous belt allows the process of theinvention to be carried out in a continuous manner with the resultantbenefits in terms of production time and cost.

In one preferred aspect, when the molten metal droplets are ejectedvertically downwards, the solid release layer moves horizontally.Preferably the solid release layer moves at a rate of 0.1 to 3.0metres/sec, such as at least 1.0 metres/sec, or at least 1.5 metres/sec.In a preferred aspect the solid release layer moves at a rate of about1.5 metres/sec.

In a further embodiment, a thin layer of a high boiling recovery liquid,such as a silicone oil, may be passed across the surface of the solidrelease layer, which may itself be mobile or static.

According to this first embodiment, process (a) preferably furthercomprises the step of releasing the metal flakes from the solid releaselayer. This may be achieved by mechanical means, such as use of a doctorblade or ultrasonics, by washing the solid release layer with a recoveryliquid optionally at high pressure, by applying a jet of gas such as airor an inert gas to the solid release layer, or by dissolving the releaselayer as previously described. A combination of these techniques mayalso be used.

Thus, in one aspect the metal flakes are released from the solid releaselayer by washing with a recovery liquid. At first sight, water appearsto be a satisfactory recovery liquid. It is readily available,inexpensive and non-flammable. However, a difficulty may arise in thecase of the more reactive metals, especially aluminium, which is themost widely used of all metal pigments. Finely divided aluminium metalmay react with water to form aluminium oxide and hydrogen gas and thepigmentary properties of the metal flake are impaired. If water is to beused as a recovery liquid for reactive metal flakes, one or morewell-known passivating agents may be required, dissolved or dispersed inthe water.

As an alternative to water, an organic liquid may be used, providing ittoo does not adversely react with the metal being processed. Use of aliquid compatible with the envisaged application, for example a surfacecoating, is particularly cost-effective. To avoid ignition of theorganic recovery liquid, it may be necessary to hold the recoveryapparatus in an atmosphere of inert gas, such as nitrogen, or under atleast a partial vacuum.

In one embodiment the liquid collecting substrates described herein maybe used as recovery liquids.

Process (b)

According to a second embodiment the present invention provides aprocess of preparing metal flakes, the process comprising the steps ofejecting molten metal from a jet head and collecting droplets of metalin the form of metal particulates in or on a collecting substrate andsubsequently milling the metal particulates to provide metal flakes.

This second embodiment may advantageously be used to prepare metalparticulates having a low median particle diameter and/or a narrowparticle size distribution. Such metal particulates form desirable,substantially monodisperse metal flakes.

The collecting substrate of process (b) may be a solid or a liquid.Preferably the collecting substrate is a liquid. Examples of suitableliquid collecting substrates include water, aliphatic and aromatichydrocarbons, such as white spirit; alcohols, ketones, esters and simpleand complex glycol ethers and esters. Particular examples of suitableliquid collecting substrates include medium or high boiling pointmineral oils, high boiling point glycol ethers such as dipropyleneglycol propyl ether, dipropylene glycol butyl ether, tripropylene glycolmethyl ether, tripropylene glycol propyl ether and tripropylene glycolbutyl ether.

Organic liquids are particularly suitable as collecting substrates, inparticular white spirit, mineral oil having a boiling point above 150°C. or propylene glycol ether having a boiling point above 150° C. It isadvantageous to use a liquid collecting substrate that is compatiblewith the envisaged application, for example a surface coating, since themetal particulates may often be safely milled directly in the liquidcollecting substrate.

When the collecting substrate is a liquid, the metal particulates aretypically metal spheres.

When the collecting substrate is a liquid, process (b) preferablycomprises the step of concentrating the metal particulates by at leastpartial removal of the liquid collecting substrate prior to milling.

When the collecting substrate is a solid, it is preferably a solidrelease layer as described herein.

As previously explained, rapid cooling takes place as the droplets ofmetal leave the jet head. According to process (b) of the presentinvention, the distance between the jet head and the collectingsubstrate is selected such that the droplets of metal are substantiallysolid when they reach the collecting substrate. The distance may be aslong as is convenient to convey the metal droplets to the collectingsubstrate. Various factors need to be taken in to account in order toselect an appropriate distance. These factors include the nature of themetal, the molten metal temperature in relation to its melting point,the droplet size, the collecting substrate temperature and theatmospheric temperature in the region of the jet head.

In a preferred aspect, the distance between the jet head and thecollecting substrate in process (b) is at least 0.20 metres, such as atleast 0.25 metres or at least 0.30 metres.

In one preferred aspect, the distance between the jet head and thecollecting substrate in process (b) is from 1 to 10 metres.

According to one embodiment molten metal is ejected from the jet head,optionally under pressure, at the top of a tall chamber filled with airor an inert gas, or held under a partial vacuum. As the molten metalfalls under gravity, it cools to a solid, such that at the base of thechamber, it falls into a liquid collecting substrate. The resultantmetal particulates may be collected and removed at the conclusion ofthis step. Alternatively, they may be continuously removed from thechamber in the liquid stream, separated, for example in a filter press,and the liquid collecting substrate recycled back into the chamber,whilst continuing to generate fresh metal particulates. In either casethe metal particulates of narrow particle size distribution areconverted into metal flakes of a narrow particle size distribution bymilling.

In order to limit the height of the chamber that is required, any gasused may be chilled and/or directed vertically upwards within thechamber, though care must be taken to ensure that adjacent moltendroplets do not impinge on each other to form aggregates. Suchaggregates will significantly change the virtually monodisperse natureof the powder particle size distribution. Additionally, it may bepreferable to employ inert gas, rather than air, in the chamber to limitor avoid oxidation of the metal particulates. Such oxidation can bedeleterious to colouristic performance when the metal particulates areconverted into metal flakes that are used in a flake pigment.

Milling

As previously mentioned, process (b) includes the step of milling themetal particulates to provide metal flakes. However milling may also becarried out in process (a).

The term “milling” as used herein includes any mechanical work performedon the particulates by moving milling media, for instance, byconventional ball milling, and alternatively, by roll milling, such aswith a nip roll.

Milling may be by a dry process or a wet process, for example in a ballmill. If a wet process is used, it may be advantageous from a costperspective to mill the metal particulates or metal flakes whilst theyare still in either a liquid collecting substrate or a recovery liquid.

Although the physical form of the metal flakes obtained from process (a)is good and they are suitable for use as pigment without furtherprocessing, for maximum brightness they may be gently milled or polishedto increase surface reflectance.

As part of process (a), milling may be carried out before or after themetal flakes are released from the solid release layer. For example, thestill molten metal droplets may be allowed to impinge directly on themoving rolls of a two or three roll mill whilst still on the solidrelease layer. When the droplets have solidified into metal flakes thesemay then be released from the solid release layer in the mannerpreviously described.

The nip between the rolls is set to impart pressure on the droplets orflakes, forcing them to assume the contours of the rolls. The surfacequality and hence the reflectivity of the resulting metal flakes iscritically dependent on the degree of surface polish of the rolls. It isan advantage of this process that as the molten metal droplets are beinguniformly generated in space and time, no overlap or cold welding ofparticles should take place, providing the rolls are rotating at asuitable speed.

Further Treatment

As previously mentioned, the metal flakes prepared according to process(a) may be released from the solid release layer by washing with arecovery liquid. A further advantage of the process of the presentinvention is the ability to treat the metal flakes whilst in therecovery liquid. Similarly, the metal particulates prepared according toprocess (b) may be collected in a liquid collecting substrate and milledto provide metal flakes. The metal flakes may then be further treatedwhilst still in the liquid collecting substrate. Alternatively dry metalflakes prepared by the process of the invention may be added to asuitable liquid for further processing.

The metal flakes in a recovery liquid, a liquid collecting substrate oranother suitable liquid may be treated for a variety of purposes. Forexample, the metal flakes may optionally be treated with ammoniumdichromate, or coated with silica or alumina, to improve stability inaqueous application media. Other treatments may be used to providecoloration of the flake surface, for example to simulate gold. Stillfurther treatments may improve the hardness and therefore the shearresistance of such flakes in application media.

In some circumstances it may be desirable to passivate the metal flakes.This may be particularly desirable when the metal flakes are to be addedto surface coating binders dissolved or dispersed in water, solvent ormixtures of the two, to prepare a surface coating, such as an ink orpaint. The reaction of certain metal flakes, notably aluminium flakes,is unpredictable. Where such a surface coating contains a proportion ofwater, there exists the possibility that reactions may occur duringstorage, with the formation of hydrogen gas with the aforementionedhazards.

Passivation of the metal flakes may be achieved through the addition ofone or more corrosion inhibiting agents to the recovery liquid, theliquid collecting substrate or another suitable liquid at any suitablepoint during preparation of the metal flakes.

Any compounds capable of inhibiting the reaction of the metal with watermay be employed as corrosion inhibitors. Examples are phosphorus-,chromium-, vanadium-, titanium- or silicon-containing compounds. Theymay be used individually or in admixture.

For certain applications, it may be necessary to concentrate the metalflakes either in the recovery liquid or in the liquid collectingsubstrate, for example to form a conventional metal flake pigment paste.Where this is the case, a filter press or other well-known means ofseparating solid particulates from liquid may be employed. To render theproduct of the process of the invention compatible with plastics andcertain printing inks, it is preferable to avoid a processing liquid,either by dry recovery of the metal flakes or through their conversioninto a liquid free form, such as granules, using for example the processdescribed in European Patent 0134676.

Metal Flakes

The process of the present invention may advantageously be used toproduce metal flakes having low median particle diameters and/or narrowparticle size distribution. The metal flakes may have functional as wellas aesthetic applications.

Metal flakes obtained or obtainable by the present invention may be usedto produce surface coatings with desirable properties. Thus in oneaspect, the present invention provides a surface coating comprisingmetal flakes obtainable by the process of the present invention. Thesurface coating may be for example an ink, a paint or a powder coating.

Metal flakes obtained or obtainable by the present invention may also beused as a metal pigment, for instance in a composition comprising themetal flakes and at least one liquid dispersant or in a liquid free formsuch as granules comprising the metal flakes and at least one solidorganic carrier material.

Metal flakes for incorporation in the metal pigment of the presentinvention may advantageously be prepared according to the process of thepresent invention.

In addition to metal flakes having a substantially circular face, it ispossible to produce other shapes such as squares, triangles and rods orbars using the processes of the present invention. Such shapes may beuseful as metal pigments, in which case the foregoing limitations andpreferences for particle size and particle size distribution will apply.However metal particles of spherical, cylindrical or other shapes mayhave further applications; for instance metal rods or bars printed bythe processes of the present invention are useful in conductors orconducting coatings. Such rods or bars preferably have a width of 2 μmor more and a length of less than about 1000 μm (1 mm). They may be usednot only for their reflectance properties but also for theirconductivity for example in electrically conductive applications, suchas EMI shielding. Such rods or bars may be prepared by reducing thespeed of the moving substrate such that the jetted molten dropletspartially overlap to form the desired shapes directly. Alternatively,they may be deposited as continuous strips, which are subsequentlyrecovered and comminuted to rods or bars.

Metal Pigment

In a further aspect, the present invention provides a metal pigmentcomprising metal flakes having a median particle diameter of 100 μm orless and a particle size distribution such that at least 90% by volumeof the metal flakes have a particle diameter within ±25% of the medianparticle diameter.

The term “median particle diameter” as used herein refers to a volumemedian particle diameter.

Particle size distributions are measured with a “Malvern Master Sizer X”which is a standard instrument for measuring volume percent particlesize distributions.

In a broad aspect, the present invention provides a metal pigmentcomprising metal flakes having a median particle diameter of 200 μm orless, preferably 150 μm or less and a particle size distribution suchthat at least 90% by volume of the metal flakes have a particle diameterwithin ±25% of the median particle diameter.

Preferably the metal flakes have a median particle diameter of 50 μm orless, such as 30 μm or less.

If the metal pigment is to be used in a surface coating it is preferablethat the metal flakes have a median particle diameter of 50 μm or less,for instance 30 μm or less, for example in the range 5 to 25 μm. If themetal pigment is to be used in an ink it is preferable that the metalflakes have a median particle diameter of 30 μm or less, for instance 20μm or less, for example in the range 5 to 15 μm.

In one aspect the metal flakes have a median particle diameter of 2 μmor more, such as 3 μm or more, or 5 μm or more.

Preferably the metal flakes have a median particle diameter in the range2 to 50 μm, preferably 5 to 30 μm, more preferably 5 to 25 μm.

Preferably the metal flakes have a particle size distribution such thatat least 95% by volume of the metal flakes have a particle diameterwithin ±25% of the median particle diameter, such as within ±20%, or±15%, or ±10%, or ±5%.

In one preferred embodiment, the metal flakes have a particle sizedistribution such that at least 95% by volume of the metal flakes have aparticle diameter within ±3% of the median particle diameter.

The aspect ratio of the flakes is preferably at least 15:1. Morepreferably the aspect ratio is from about 30:1 to about 100:1. Higheraspect ratios are generally preferable and particles having an aspectratio of 150:1 or above may be obtained by the present invention. Theaspect ratio is defined at the ratio of the largest dimension to thesmallest dimension. In a preferred aspect the metal flakes have asubstantially circular face.

Apparatus

In one aspect the present invention provides an apparatus for producingmetal flakes, the apparatus comprising (i) a jet head comprising atleast one nozzle; and (ii) a collecting substrate arranged to collectmolten metal droplets ejected from the jet head characterised in thatthe collecting substrate is a solid release layer.

Jet heads of the continuous ink jet (CIJ) and drop-on-demand (DOD) typesare especially amenable to the process and apparatus of the invention,provided that the piezoelectric control of the latter is kept wellinsulated and so well below the temperature of the molten metal. Aplurality of nozzles is desirable in order to prepare commercialquantities in a reasonable timescale. A filter for the molten metalbefore it enters the nozzles is also advantageous. For example, a filtermay be desirable to avoid blocking the nozzle(s) if there areimpurities, such as oxide or slag, in the molten metal.

Preferably at least the jet head and the solid release layer are in achamber under an inert atmosphere, or under at least a partial vacuum.

As mentioned above, a suitable solid release layer is PTFE, silicon,polished glass or polished ceramic. In a preferred aspect the solidrelease layer is or is on a continuous belt.

Preferably the distance between the jet head and the collectingsubstrate is less than 0.30 metres, such as less than 0.25 metres orless than 0.20 metres. More preferably the distance between the jet headand the collecting substrate is from 0.001 to 0.01 metres.

The invention is further illustrated by the following Examples in whichall parts and percentages are by weight.

EXAMPLES Example 1

Metallic tin was melted in the reservoir of a jet head, fabricatedmainly from stainless steel and having 120 circular nozzles, each of 35μm diameter, drilled in a 75 mm diameter, 380 μm thick silicon waferattached thereto. At an operating frequency of 4,000 Hz, the molten tindroplets were allowed to fall 0.7 cm onto a PTFE belt, movinghorizontally at 0.45 metres/sec. Solidified tin flakes were continuouslyremoved from the belt after the deposition station by washing with a fanjet of mineral spirits in an atmosphere of nitrogen. The thus collectedflakes were concentrated in a filter press to give a metal pigment pastehaving a solids content of 90% by weight. A solvent-based paint preparedfrom the metal pigment paste demonstrated excellent brightness and asilver tone with a very pale gold tinge.

Example 2

Metallic tin was melted in the reservoir of a jet head. At an operatingfrequency of 3,000 Hz and with 40 psi pressure of nitrogen gas, themolten tin was forced through multiple 20 μm nozzle orifices verticallydownwards from the top of a 2.5 m high column, inerted by nitrogen gas.Solidified tin spheres were allowed to fall into a shallow mass of whitespirits solvent at the base of the column. The thus collected powder wasconcentrated in a filter press to give a filter cake having a solidscontent of approximately 90% by weight. The variation in diameter of thecollected material was a maximum of only +/−4%.

33.0 kg of the thus prepared filter cake,

0.5 kg oleic acid and

50.0 kg white spirits were milled in a ball mill with 450 kg of 3.5 mmdiameter steel balls for 3 hours. The flake pigment obtained was removedfrom the mill by washing with further white spirit and collected in afilter press. The variation in diameter of the material, collected invirtually quantitative yield was less than 5%.

A solvent-based paint prepared from the resulting metal flake pigmentpaste demonstrated outstanding brightness and a silver tone with a verypale gold tinge.

Example 3

A jet print head is constructed to demonstrate the concept. The printhead includes an integrally fitted reservoir for the molten metal thatis machined from molybenum. Sealing to the top and bottom plates is bymeans of flexible graphite gaskets. Heating of the metal is by anelectrical resistance unit with an integral thermocouple formed into aspiral to fit tightly outside the reservoir. The bottom plate is aceramic disc with a 1 mm hole in the centre. A laser drilled ruby nozzlewith a diameter of 20 μm is cemented into the centre of this disc. Amolybdenum piezoelectric driven ruby diaphragm bonded to a ceramic formsthe top plate of the reservoir. Insulation is fitted between thereservoir heater and the top and bottom plates of the print head. Themolten aluminium is passed through a ceramic filter before entering theprint head reservoir.

Examples 1 and 2 are repeated using this jet print head.

TABLE 1 Size (μm) Vol Under % 0.020 0.00 0.022 0.00 0.025 0.00 0.0280.00 0.032 0.00 0.036 0.00 0.040 0.00 0.045 0.00 0.050 0.00 0.056 0.000.063 0.00 0.071 0.00 0.080 0.00 0.089 0.00 0.100 0.00 0.113 0.00 0.1260.00 0.142 0.00 0.159 0.00 0.178 0.00 0.200 0.00 0.224 0.00 0.252 0.000.282 0.00 0.317 0.00 0.356 0.00 0.399 0.00 0.448 0.00 0.502 0.00 0.5640.00 0.632 0.00 0.710 0.00 0.796 0.00 0.893 0.00 1.002 0.00 1.125 0.001.262 0.00 1.416 0.00 1.589 0.00 1.783 0.00 2.000 0.00 2.244 0.00 2.5180.00 2.825 0.00 3.170 0.03 3.557 0.10 3.990 0.22 4.477 0.40 5.024 0.655.637 0.99 6.325 1.45 7.096 2.04 7.962 2.78 8.934 3.70 10.024 4.8211.247 6.16 12.619 7.77 14.159 9.68 15.887 11.95 17.825 14.66 20.00017.88 22.440 21.68 25.179 26.14 28.251 31.28 31.698 37.09 35.566 43.5239.905 50.43 44.774 57.64 50.238 64.91 56.368 71.96 63.246 78.58 70.96384.46 79.621 89.45 89.337 93.42 100.237 96.35 112.468 98.32 126.19299.44 141.589 99.93 158.866 100.00 178.250 100.00 200.000 100.00 224.404100.00 251.785 100.00 282.507 100.00 316.979 100.00 355.656 100.00399.053 100.00 447.744 100.00 502.377 100.00 563.677 100.00 632.458100.00 709.627 100.00 796.214 100.00 893.387 100.00 1002.357 100.001124.683 100.00 1261.915 100.00 1415.892 100.00 1588.657 100.00 1782.502100.00 2000.000 100.00

1.-32. (canceled)
 33. A metal pigment comprising metal flakes having amedian particle diameter of 100 μm or less and a particle sizedistribution such that at least 90% by volume of the metal flakes have aparticle diameter within ±25% of the median particle diameter.
 34. Ametal pigment according to claim 33 comprising metal flakes having amedian particle diameter of 50 μm or less.
 35. A metal pigment accordingto claim 33 comprising metal flakes having a median particle diameter of30 μm or less.
 36. A metal pigment according to claim 33 comprisingmetal flakes having a median particle diameter of 20 μm or less.
 37. Ametal pigment according to claim 33 comprising metal flakes having amedian particle diameter in the range 5 to 15 μm.
 38. A metal pigmentaccording to claim 33 comprising metal flakes having a particle sizedistribution such that at least 95% by volume of the metal flakes have aparticle diameter within ±25% of the median particle diameter.
 39. Ametal pigment according to claim 33 comprising metal flakes having aparticle size distribution such that at least 95% by volume of the metalflakes have a particle diameter within ±3% of the median particlediameter.
 40. A metal pigment according to claim 33 wherein the metalflakes have an aspect ratio of at least 15:1.
 41. A metal pigmentaccording to claim 33 wherein the metal flakes have an aspect ratio ofabout 30:1 to about 100:1.
 42. A metal pigment according to claim 33wherein the metal flakes have an aspect ratio of at least 150:1.
 43. Ametal pigment according to claim 33 wherein the metal flakes have asubstantially circular face.
 44. A metal pigment according to claim 33which is obtainable by a jetting process comprising the steps ofejecting molten metal from a jet head, wherein the molten metal is inthe form of droplets which are induced to part reliably and uniformly onegress from the nozzle entrance, and either: (a) collecting droplets ofmetal on a collecting substrate, wherein the collecting substratecomprises a solid release layer; or (b) collecting droplets of metal inor on a collecting substrate and subsequently milling the collectedmetal droplets.
 45. A surface coating comprising a metal pigment asdefined in claim
 33. 46. A surface coating according to claim 45 whichis an ink, a paint, a powder coating or a metal flake pigment paste.